A fine tool servo system for global position error compensation for a miniature ultra-precision lathe

There is an increasing demand for single-point diamond turning to manufacture micro components as well as micro features on a large workpiece surface. In order to obtain high accuracy and a fine surface finish of the large area workpiece, position control of machine tool has become the main concern to achieve the high precision position control. A coarse-fine servo system is able to provide a cost-effective solution. This system can provide information on the entire guidance errors profile data and simultaneously compensate the error in real-time by using the fine position control technique. In this study, a piezoelectric actuator based fine tool servo (FTS) system has been developed and it has been incorporated with a miniature ultra-precision lathe. A cost-effective position sensitivity detector (PSD) is integrated in the FTS design, which is able to measure the global straightness error of the translational slide accurately. The detected error signals are compensated by the FTS during the turning process. For better tracking performance, a proportional-integral (PI) feedback controller has been implemented and tested in this study. Experimental results show that the developed FTS can effectively and successfully compensate the micro waviness error which is caused by the x-axis translational slide of the miniature ultra-precision lathe.     

Microstructure of fast tool servo machining on copper alloy

The development of the fast tool servo (FTS) for precision machining was investigated.The micron machining performance of a piezoelectric-assisted FTS on copper alloy was evaluated.The results indicate that the quality of the microstructure depends mainly on two important factors:the cutting speed (or spindle speed) and the driving frequency of the FTS.The excessive driving frequency increases the formation of burrs.The effect of the clearance angle of the diamond tool on the microstructure machining precision was also investigated.     

MPCVD多晶金刚石片的研磨均匀性分析

在游离磨料研磨过程中,研磨的驱动方式及工艺参数等直接影响加工后工件的平面度和表面粗糙度。为了探究基于旋摆式驱动的游离磨料研磨工艺参数对MPCVD多晶金刚石片平整化的影响,建立旋摆式驱动平面研磨过程中的单磨粒运动学模型,根据实际研磨过程采用多磨粒随机分布模型进行计算机仿真计算,引入多磨粒轨迹的均匀性离散系数对磨粒轨迹均匀性进行分析。结果表明:当转速比取值等于0.5时,磨粒轨迹离散系数最大;当转速比小于等于0.5时,离散系数与转速比为正相关;研磨盘摆动弧线的弦长大于金刚石片直径时,磨粒相对于整个金刚石片表面的运动轨迹分布较为均匀;计算机仿真计算得到了研磨最优参数,并通过2英寸MPCVD多晶金刚石片研磨试验验证了仿真结果的有效性。研磨后金刚石片表面PV值为2.4 μm,表面粗糙度R_a达到139 nm,材料去除率d_MRR为10.1 μm/h。      

FEM optimization of tool geometry based on the machined near surface''s residual stresses generated in diamond turning

In this work, based on the updated Lagrangian formulation and the commercial available software, Marc2001, a coupled thermo-mechanical plane-strain large deformation orthogonal cutting FE model is presented to simulate the diamond turning process and predict the residual stresses on the machined surface of workpiece. In order to consider the interactive influences of cutting edge radius, cutting velocity, rake angle and clearance angle on residual stresses, all simulations are programmed by an orthogonal design method, i.e. the combination design of general rotary method. As expected, two regression equations of tensile and compressive residual stresses are deduced according to the simulated results. The measured results in diamond turning show that the predicted results have a good consistency with the experimental ones. Therefore, some related analyses are carried out for the influencing factors based on the regression equations. Finally, the optimal analyses indicate that a rake angle of 15° and a clearance angle of 10° are the optimum geometry of a diamond tool in turning of ductile materials when this tool has a cutting edge radius of 100–300 nm.     

回转件电解车削加工基础工艺试验研究

讨论了数控电解车削应用于难加工材料回转件加工的可行性,利用直线刃阴极在电解车削加工装置上进行了圆柱面回转件的加工试验,得到了主轴转速、加工间隙、工作电压相互之间的关系曲线,并对此进行了分析。     

UMAC时基控制原理及其在非轴对称微结构表面切削中的应用

针对非轴对称微结构表面加工的实时性要求,采用压电陶瓷驱动的快速伺服刀架(FTS)作为微进给机构,实现刀具沿z向高频响、短行程的快速精密进刀运动,并构建了基于UMAC多轴运动控制器的微结构车削数控系统。应用UMAC的时基控制功能实现快速伺服刀架(FTS)进给与主轴转角θ的同步,来完成非轴对称微结构表面的加工。     

A novel spindle inclination error identification and compensation method in ultra-precision raster milling

In the ultra-precision raster milling (UPRM) process, the existence of spindle inclination error can directly affect the dimensional accuracy of machined components. This study developed a novel spindle inclination error identification and compensation method based on the groove cutting in UPRM. In this method, the tilt angle of the intersection curve of two toruses (ICTT) generated from two neighboring rotary cuts in UPRM was measured to identify the spindle inclination error. A mathematical model was developed to simulate the ICTT profile and present the relationship between the tilt angle of ICTTs and the spindle inclination error by solving the differential of the ICTT function, by which the spindle inclination error can be solved under the given cutting parameters and the tilt angle of ICTTs. The effects of cutting parameters on the tilt angle of ICTTs were explored. An error compensation procedure was designed and a group of groove cutting experiments was conducted to identify and compensate the spindle inclination error. The theoretical and experimental results show that the proposed method can compensate for the spindle inclination error effectively and accurately.     

Signal analysis of surface roughness in diamond turning of lens molds

Diamond turning of high-precision lens molds is an important production process. The surface roughness of the mold heavily affects the quality of lens. In diamond turning, the surface roughness obtained depends on the cutting tool, the cutting conditions, the machine characteristics, the surrounding vibrations and the work piece material. This work studies the surface roughness obtained from the diamond turning of a phosphor–bronze lens mold with various tool nose radii, spindle speeds, feed rates and cutting depths. The surface roughness was measured in the time domain using a Form Talysurf instrument (a stylus-type surface roughness meter) and then transformed into the frequency domain using the fast Fourier transform. Based on the magnitude of the intensity, the tool geometry, low-frequency vibration and the measuring instrument are identified as the main influencing factors of the generated surface roughness. The intensities associated with the latter two vary little with the cutting conditions and are thus considered constant. The intensity of the tool geometry varies with the feed rate, the spindle speed and the radius of the tool nose. A relationship between the root-mean-square summation of the surface roughness and cutting conditions was found. The model agrees well with the experimental results. The analysis also identified the critical feed rate that maximized machining productivity, below which the surface roughness was only slightly improved as the production rate fell sharply.     

Application of active disturbance rejection control to variable spindle speed noncircular turning process

In application of variable spindle speed machining to noncircular turning process, the tracking control of the fast tool servo yields a periodic sampling rate in the real-time domain. However, in the view of the angle domain, the sampling rate is constant. Moreover, the reference acceleration signal is easily available. This implies that it is advantageous to design the control system from the angle domain perspective, but the plant for a linear time-invariant system will become a periodically time-varying system. In this paper, the active disturbance rejection control strategy is applied to actively estimate the time-varying dynamics and other disturbances and compensate for them in the control law. The acceleration feed-forward strategy is employed to achieve better tracking performance. The stability analysis based on the lifting technique is proposed. Experimental machining results demonstrate the tracking performance of the proposed design, as well as the ability to increase machining stability using variable spindle speed machining.     

Feed-forward control of fast tool servo for real-time correction of spindle error in diamond turning of flat surfaces

A fast tool servo is designed and tested to obtain sub-micrometre form accuracy in diamond turning of flat surfaces. The thermal growth spindle error is compensated for real time using a fast tool servo driven by a piezoelectric actuator along with a capacitive displacement sensor. To overcome the inherent non-linearity of the piezoelectric actuator, Proportional Integral (PI) feedback control with a notch filter is implemented. Besides, feed-forward control based on a simple feed-forward predictor is added to achieve better tracking performance. Actual machining data are discussed in detail to prove that the proposed fast tool servo is capable of fabricating flat aluminum specimens of 100 mm in diameter to a form accuracy of 0.10 μm in peak-to-valley error value.     

基于Matlab磨料轨迹仿真的磨盘地貌优化设计

为设计节块式磨盘地貌、提高金刚石磨盘的磨削加工质量,从研究磨削轨迹特征入手,利用Matlab软件对节块式磨盘磨料的运动轨迹进行仿真。以磨料磨削轨迹密度为评价依据,通过仿真正交试验优化了电机转速、工件进给速度、磨盘节块数量等参数,然后对排布角度进行单因素优化仿真试验,并通过20钢磨削实验对仿真结果进行验证。结果表明:增加电机转速和节块数量,减小工件进给速度,适当增加排布角度能提高磨削轨迹密度;电机最佳转速为11 000r/min,工件最佳进给速度为10mm/s,节块最佳数量为18块,在上述参数条件下获得的排布角度最佳值为40°。     

Tool wear control in diamond turning of high-strength mold materials by means of tool swinging

A tool-swinging method was proposed to reduce tool wear in diamond turning of high-strength mold materials. A round-nosed diamond tool was swung by rotating the B-axis rotary table of the machine, the center of which was aligned with the tool center. The tool-decentering error was detected and compensated for by an on-machine measurement system. The effects of tool-swinging direction, swinging speed, lubricant type, and tool rake angle were investigated. The tool wear was greatly reduced compared to the conventional method. A surface finish of 4 nm Ra was obtained on reaction-bonded silicon carbide by generating continuous chips.     

Improving the shape quality of bearing rings in soft turning by using a Fast Tool Servo

In machining of ring shaped components, the workpiece is deformed by the clamping forces of the chuck. This elastic deformation generates shape deviations in soft turning. Moreover, the machining process generates locally varying residual stresses which contribute to shape deviation of the workpiece. Hence, in machining of thin-walled bearing rings hexagonal out‐of‐roundness up to 200 μm occur. In order to minimize the shape deviations, a long stroke Fast Tool Servo (FTS) for controlling the depth of cut was developed. The applied FTS differs from other published FTS systems in the guidance design. The moving tool holder is suspended to the FTS frame by flexure joints instead of using a linear guidance. The flexure joints provide a low stiffness in moving direction and high stiffness in orthogonal directions. The high stiffness in cutting force direction is essential for a real time reduction of shape deviations in soft turning. In this paper, results of an experimental investigation for the reduction of the shape deviation by adapted non circular machining are presented, using the developed FTS. Based on the results, the influence of the cutting forces on part accuracy is discussed.     

Dynamic rotating-tool turning of micro lens arrays

Diamond micro milling of high-quality micro lens arrays suffers from low machining efficiency, due to the inevitable milling marks along tangential feed direction and the slow spiral tool path interpolated by multiple linear axes. In this article, an advanced cutting process is proposed, namely dynamic rotating-tool (DRT) turning, in which a U-axis attachment on a rotary stage is developed to enable synchronous cutter rotation and radial feed motions of a diamond turning tool. This method is experimentally verified and compared with milling, with significantly enhanced surface quality and machining efficiency, thus bringing a new perspective into ultra-precision machining.     

Development of a new rotary ultrasonic spindle for precision ultrasonically assisted grinding

In our previous work, a new method for inducing a machine spindle to ultrasonically vibrate was proposed in which the axial ultrasonic vibration is excited by a fluctuating electromagnetic force applied to the spindle. The validity of this method was also confirmed experimentally. In this paper, focusing on the development of a new rotary ultrasonic spindle based on this new method, an actual ultrasonic spindle unit was designed and constructed in accordance with the previous work. The unit consists of an ultrasonic spindle, two rotary bearings and their housings for holding the spindle, eight electromagnets and their supporters for the generation of a fluctuating electromagnetic force, and a base plate. Its performance was also investigated experimentally for different exciting conditions. The results showed that an ultrasonic vibration with a sub-μm order amplitude is generated on the produced spindle and that the applied electromagnetic force affects the spindle performance significantly.     

单点金刚石车削的工艺参数对表面粗糙度影响的实验研究

超精密车削中单点金刚石刀具切削参数的设置非常重要,这不仅关系到金刚石刀具的使用寿命,而且对于提高车削效率以及得到更好的工件表面车削质量都有着深远的意义.文章着眼于实验,首先对超精密车削中切削量、刀尖圆弧半径、进给量和主轴转速对表面微观形貌的影响进行了实验研究.然后对实验的数据结果进行了系统的归纳整理,最后分析了这些影响所产生的原因.根据实验的研究及分析的结果,可得到超精密加工切削参数的优化组合,为提高实际车削后工件的表面粗糙度提供了依据.     

Improving waviness in ultra precision turning by optimizing the dynamic behavior of a spindle with magnetic bearings

The aim of this paper is to present an approach of improvement of the surface flatness in diamond turning with a spindle on magnetic bearings. The procedure followed to attenuate the defect involves two processes of dynamic compensation of the spindle parasitic movements: (i) the classical dynamic balancing of mainly the tilting motion; (ii) the more specific active compensation of the non-linear components of the spindle movement by minimization of the harmonics of the spindle displacements synchronous with the frequency of rotation. Surface analysis at each step shows that the evolution of the surface figure is closely correlated, at a scale of some nanometers, with the dynamic movements of the spindle, and may consequently be significantly attenuated with an active compensation.     

旋转编码器在压力机双移动工作台上的应用

通过在工作台上加装旋转编码器,检测工作台的实时位置,利用数据采样及比较功能实现工作台的自动减速、到位停止,实现两个移动工作台的防碰撞功能。利用先进的现场总线将编码器与其他联网设备连接,数据采样更加迅捷,给出了移动工作台自动运行的一个优化设计方案。为机械压力机的自动换模奠定了坚实基础。     

Identification of tool parallel axis offset through the analysis of the topography of surfaces machined by peripheral milling

A method for the identification of the tool parallel axis offset (TPAO) that occurs when the end mill is held in the spindle is developed. The method is based on the analysis of the topography of surfaces machined by peripheral milling and considers the cutter grinding errors. As known from the literature, TPAO causes each cutting edge to be at a different radius from the spindle axis and creates transition bands in the topography of milled surfaces, in which roughness grooves generated by different teeth blend together. In this paper, the TPAO, defined by the distance of the tool axis from the spindle axis and by an angle relating the offset direction to the position of cutting edges, is expressed as a function of the width of the roughness grooves at any height of the transition bands. This expression allows the TPAO to be estimated by measuring the groove widths at only two heights and solving a system of two linear equations. In order to obtain the groove widths, a procedure based on digital image processing is developed. Through this procedure, the groove widths are estimated at more than the two necessary heights without high computational cost. This leads to the resolution of an overdetermined system of linear equations that allows the TPAO to be identified with more accuracy. Finally in order to verify the predictions of the proposed method, a series of cutting tests were carried out. A reasonable agreement between the experimental results and the predicted ones was found.     

万能实验机电机速度和位移的闭环测量与控制

采用AVR单片机作为控制核心，步进电机带动丝杠运动，光栅编码器测量位移，在材料万能实验机上建立了一种闭环控制系统。